This invention relates to an insert designed to be fitted within a fireplace in order to form a furnace which will burn a solid fuel, such as firewood or coal, for use in home heating. The invention disclosed in this application provides a number of features not shown in the prior art which, collectively, permit the furnace to make efficient use of the solid fuel being burned. Despite the relatively high heat output, a novel arrangement of tubular heating ducts and an outer heat exchange chamber prevent overheating of the furnace and of the inside walls of the fireplace itself. Furthermore, the inclusion of large glass doors on the furnace permit the fire to be viewed while retaining the efficiency of the furnace, or, if desired, viewed with the doors open so that the furnace functions essentially as a fireplace but with the ability to recover and dispense into the room substantial heat which would otherwise be lost.